Solar power systems offer much promise for clean energy, with few, or zero, carbon emissions. These systems collect incident sunlight and convert this sunlight into a usable form of power, such as heat or electricity. Solar energy offers a clean, inexhaustible, sustainable solution to energy demands and has the potential to supply a very significant fraction of U.S. and global electricity consumption. While the U.S. and global solar power potential is known to be immense, solar power systems have not been economically competitive without government support, to date. Challenges remain to devise solar technologies that can lower installation costs, increase power output, and lower the marginal cost per unit energy produced, for a lower levelized cost of energy. An important metric is the overall system efficiency, that is, the electric power output per incident solar power collected.
Solar power systems include photovoltaic (PV) systems, solar thermal systems, and others. PV systems utilize photovoltaic solar cells that convert sunlight directly into electricity by the photovoltaic effect. These solar cells are expensive, and their efficiencies are limited because they can exploit only a portion of the solar spectrum. These systems are also characterized by a large energy-payback period, i.e., the time they must be exposed to sunlight and produce electricity, to return the energy required to produce and install them.
Solar thermal systems convert sunlight into heat and either use this heat directly or convert the heat to generate electricity. Examples of solar thermal systems include solar power towers, parabolic trough systems, and dish-Stirling systems. Solar power towers utilize a large number of steerable, planar, or near-planar mirrors that reflect and direct rays of sunlight to a central tower where a heat-transfer fluid is heated. The heat collected is typically transferred to rotating machinery, such as a steam turbine, that is used to drive an electric generator. These systems suffer from low efficiencies because of high optical losses, such as cosine and other optical losses, solar-receiver losses, as well as temperature and power losses from long fluid-flow loops to and from the tower. Cosine losses refer to the energy lost when light rays from the sun do not strike the mirror perpendicular to its surface. To reflect rays of sunlight to the central tower, individual mirrors form an acute angle to the sun, therefore requiring more mirror surface than when the mirror is perpendicular to the sun's rays. Collection efficiency is increased and mirror cost is less when the mirror is perpendicular to the sun.
Volumetric solar receivers have been developed and implemented in concentrating solar power towers. The objective is to irradiate a honeycomb or waffle pattern of channels while pulling air through the channels to heat the air. The air is then used to heat a storage material or to generate steam for electricity production. Current designs of the channels do not allow for deep penetration of the irradiance, and the receiver surfaces get hot near the aperture, maximizing radiative heat loss. None of the previous volumetric receiver designs integrates PV.
Solar receivers have also been used to heat particles, both inert and thermochemically reactive particles for additional energy storage. Although no commercial solid particle receivers exist, a significant amount of research has been performed to develop efficient solid particle receivers for energy storage and electricity production. None of these previous concepts has included the use of a light-transmitting PV array at the aperture to generate electricity while mitigating convective and radiative heat losses.
The need remains, therefore, for a solar thermal system that efficiently converts sunlight into heat. The need also remains for solar power systems that combine the efficiencies of solar thermal systems and PV systems. The need also remains for solar power systems that combine the efficiencies of thermochemical particle systems and PV systems.